The present studies focused on the molecular properties and photochemical reactions of visual pigments in order to elucidate how the difference in light response between rods and cones can be explained on the basis of visual pigments.1. Using a cDNA cloning technique, we have isolated four kinds of cDNA clones encoding chicken cone visual pigments and two clones encoding nocturnal gecko visual pigments. The amino acid residues responsible for the spectral tuning of the visual pigments were speculated on the basis of the differences in amino acid sequence among the visual pigments. A phylogenetic tree of vertebrate visual pigments was then constructed on the basis of amino acid identity. It indicated that an ancestral visual pigment evolved first into four groups of cone visual pigments and that group of rhodopsins, the rod visual pigments, diverged later from one of the four groups. Thus, it is suggested that animals had acquired first the ability to distinguish color and then acquired scotopic vision.2. We have investigated the bleaching process of iodopsin by means of low temperature spectroscopy and laser photolyses. The results showed that iodopsin has similar intermediates corresponding to photo-, batho-, BL, lumi, meta I-, and meta II-intermediates of rhodopsin in its bleaching prosess. Furthemore, like metarhodopsin II, metaiodopsin II has a binding ability to transducin, suggesting that it has a physiological role similar to metarhodopsin II.Comparative studies of cone visual pigments, iodopsin, chicken green and chicken blue with the rod visual pigment rhodopsin clearly showed that cone visual pigments are faster than rhodopsin in rate of regeneration from 11-cis-retinal and opsin, and those of formation and decay of physiologically active intermediate (meta II-intermediate). These difference closely correlate to faster dark-adaptation, faster light response and less photosensitivity of cones than rods, respectively.